WordNet: an online lexical reference system (dictionary and much much more) developed at Princeton University's Cognitive Science Laboratory.
http://www.cogsci.princeton.edu/~wn/
WordNetX: Mac interface to WordNet.

Either the link is dead or this is a problem with Sourceforge, which does not like direct download links. You can find a similar and equally free product at Nisus, called Nisus Thesaurus. It too is based on WordNet.